The present invention relates to a cutting tool inserts consisting of a substrate at least partially coated with a coating consisting of one or more refractory layers of which at least one layer is a MTCVD-Ti(C,N)-layer composed of grains with grain size just above the nanograined region with columnar grain morphology. As a result the problem with grain boundary sliding at higher temperatures has been reduced and, consequently, wear resistance increased. The inserts are particularly useful in applications involving carbon steels or grey cast irons where wear resistance is important.
Coated bodies used for metal cutting are well known. Typically, the bodies are made of a cemented carbide, cermet or ceramic and the coatings are one or more of a Group VIB metal carbide, nitride, oxide or mixtures thereof. For example, cemented carbides coated with layers of TiC, Ti(C,N), Al2O3 and TiN are widely used. There are many variations in layer composition and thickness. The layers are applied by various methods such as CVD (chemical vapor deposition), both conducted at normal temperatures of from about 900 to about 1100° C. and MTCVD (medium temperature chemical vapor deposition) conducted at temperatures of from about 700 to about 900° C., and PVD (physical vapor deposition).
CVD TiC coatings are usually composed of equiaxed grains with the grain size being form about 0.5 to about 1.0 microns. CVD TiN as well as MTCVD TiCN coatings are composed of columnar grains with the length of the grains approaching the coating layer thickness. The morphology of CVD coatings can be slightly modified by process adjustments. The MTCVD coatings are, however, very difficult to modify by conventional process adjustments. MTCVD coatings are particularly characterized by the presence of large columnar grains with the length of the crystals approaching the thickness of the coating layer. Ti(C,N) layers produced by using MTCVD are today almost exclusively used instead of CVD TiC or Ti(C,N).
It is well-known that the hardness of polycrystalline materials in general (including coating layers as well) obey the Hall-Petch equation: H=H°+C/√d where H is the hardness of a polycrystalline material, H° is the hardness of a single crystal, C is a material constant (C>0) and d is the grain size. As may be seen from this equation, the hardness of a material can be increased by decreasing the grain size.
This relation is, however, not necessarily correct for hard and brittle materials with limited plasticity. Furthermore, when dealing with nanograined hard materials with extremely fine grain sizes, the fraction of material in grain boundaries is increased and this effect has to be taken into consideration. Consequently, a reverse Hall-Petch dependence has been observed in many studies dealing with nanograined materials. Generally, it is assumed that the relationship is valid for grain sizes down to from about 20 to about 50 nm. At these crystal sizes, mobility and multiplication of dislocations will be severely reduced. The properties of grain boundaries will start to dominate and grain boundary sliding has been suggested to be responsible for the reverse Hall-Petch dependence.
As clear from U.S. Pat. No. 6,472,060, the crater wear resistance is reduced when the grain size is decreased in to the nanograined region even though the room temperature hardness is increased. This is explained by increased amount of grain-boundary sliding. Consequently, when wear resistance is considered there is an optimum grain size region just above the nanograined region for the maximum performance. It is emphasised that the optimum grain shape (morphology) is not the same for all work piece materials and should consequently be controlled for different cutting conditions and work piece materials. In all applications, however, the grain size should be slightly above the nanograined region.
The use of different dopants such as a tetravalent titanium, hafnium and/or zirconium compound in the formation of an Al2O3 layer to promote the formation of a particular phase is shown in U.S. Reissue Pat. 31,526. Also, the use of a dopant selected from the group consisting of sulphur, selenium, tellurium, phosphorous, arsenic, antimony, bismuth and mixtures thereof to increase the growth rate of Al2O3 applied by CVD as well as to promote even layers of the coating is disclosed in U.S. Pat. No. 4,619,886. Dopants can also be applied to refine the grain size of MTCVD coatings. The use of CO doping to achieve nanograined MTCVD Ti(C,N) layers is disclosed in U.S. Pat. No. 6,472,060.
U.S. Pat. No. 6,472,060 discloses a method where relatively high amounts of CO, from about 5 to about 10%, preferably from about 7 to about 9% of the total gaseous mixture, are used in MTCVD in order to obtain a grain size of the order of 25 nm or less, preferably 10 nm or less. The CO-doped nanograined MTCVD coatings exhibited increased toughness, however, with reduced crater wear resistance as a consequence.
It has previously been shown (U.S. Pat. No. 6,472,060) that the grain size of MTCVD coatings can be decreased considerably and brought into the nanograined region. These nanocrystalline layers should preferably be applied as outermost layers. The nanocrystalline coatings are harder at room temperature but exhibit grain boundary sliding leading to plastic deformation at higher temperatures (at higher cutting speeds).
Due to the extremely fine grain size of these coatings, the surface smoothness is increased and friction coefficient is reduced. Consequently, nanocrystalline coatings can be used as friction reducing/lubricating layers and should, as mentioned above, deposited on top of the existing coating structure.
There is consequently still need to optimise and control the grain size of MTCVD coatings with respect to the material to be machined.